In accordance with the invention, it has been discovered that optimum specific surface areas of the metal catalyst deposited on any silica carrier and small particle sizes of the metal catalyst are obtained when urea is employed in predetermined amounts to generate hydroxyl ions in the process comprising (1) increasing the hydroxyl ion concentration of a mixture of silica in water and a water soluble salt of the metal catalyst to precipitate insoluble salts of the metal catalyst on the silica, with agitation at temperatures above 50.degree. C.; and (2) reducing the insoluble salt of the metal catalyst to produce free metal catalyst deposited on the silica carrier. Optimum specific surface areas of the metal catalyst deposited on the silica carrier and small particle sizes of the metal catalyst are obtained when the predetermined amount of urea satisfies the equation. EQU U = f.sqroot.Ni (1)
where:
U = urea concentration in grams per liter; PA1 Ni = nickel ions concentration in grams per liter of water; f = a factor varying between 75/pH and 175/pH, in which pH = the pH value of the suspension of the silica in the water. PA1 U = urea concentration in grams per liter; PA1 Ni = nickel ions concentration in grams per liter of water; PA1 f = a factor varying between 75/pH and 175/pH, in which pH = the pH value of the suspension of the silica in the water.
Various silica carriers are marketed commercially. Specific silicas have specific properties. In particular, specific silicas may have characteristics pHs. For instance, the pH of AEROSIL (trade name for silicon dioxide obtained by flame-hydrolysis of silicon chloride) is about 3.5. The pH of Ketjensil (trade name for silicon dioxide obtained by subjecting water glass to neutralization with sulfuric acid and, subsequently, to spray-drying) is about 6.5. The pH of Kieselguhr, varies with kind and with source, from a pH of about 3.8 to 8.5.
In turn, the size of the specific areas of catalyst metals deposited on silicas, and particle sizes, depend on the nature of the silica carrier employed. The amount of urea used to act as a source of hydroxyl ions, when used in a process comprising increasing the hydroxyl ion concentration of a mixture of silica in water and a water soluble salt of the metal catalyst to precipitate an insoluble salt of the metal catalyst on the silica, must be determined empirically.